Impossible to hide USB devices, specifically the host system disk

Description

My host system (Ubuntu Jaunty 9.04) boots from an external USB disk that is selectable in the USB selection menu as any other connected usb device.

If I click on it by mistake it happens that the system disk gets disconnected from the host and assigned to the guest, making the system instantly unstable and unusable, and corrupting every filesystem on it.

Since this situation is not really common I don't know if this possibility should be denied in a first place, but I think that at least there should be the possibility to hide or exclude specific USB devices from the selection menu.